Control for turbine gate actuators



June 22, 1954 Filed Feb. 4, 1950 2 Shee'ts-Sheet 1 l El I I I 25 l v \9 2e, my i a; -2?

55 I: '--L=!-.- 3s 24 NIH H I Z M II E NVENTOY-I-J" Clarence L-mvev'yg Francis =3. 2 Lv-chew cA/M Er /MAW June 22, 1954 AVERY ET AL 2,681,660

CONTROL FOR TURB INE GATE ACTUATORS Filed Feb. 4, 1950 2 Sheets-Sheet 2 iNVBnrrow-LS Clarence L tflvem FW-Qncis cJ- Z LY-chew- Twpr-zmex Patented June 22, 1954 CONTROL FOR TURBINE GATE ACTUATORS Clarence L. Avery and Francis J. Zircher, Rockford, 111., assignors to Woodward Governor Company, Rockford, 111., a corporation of Illinois Application February 4, 1950, Serial No. 142,506

11 Claims. 1

This invention relates to an automatic control for limiting the rate of closure of the gates of a hydraulic turbine or water wheel. Such gates are actuated by a power servo which is also utilized to operate an auxiliary valve by which water may be by-passed around the turbine to prevent damage to the water conduit system or turbine due to excessively high water pressures caused by closure of the gates at an excessive rate. The opening and also the reclosing of the by-pass valve is determined by the action of an adjustable dashpot.

The primary object of the present invention is to provide a supplemental control for a gate actuating servo of the above character to avoid closure of the gates at an objectionably high rate in the event that the by-pass valve fails to operate properly.

Another object is to limit the rate of servo motion in closing the gates if this rate rises sufficiently above the intended rate of opening of the by-pass valve.

A further object is to adapt the supplemental control for operation in the intended manner in various positions of the gates while disabling the control at the normal or permissible rates of gate travel.

Still another object is to utilize the regular governor control relay valve in a novel manner to obtain the supplemental or safety control action above referred to.

Other objects and advantages of the invention will become apparent from the following detailed description taken in connection with the accompanying drawings, in which Figure 1 is a schematic view of a turbine gate actuator equipped with the improved rate limiting control.

Fig. 2 is a fragmentary view of the servo relay valve in a different position.

Fig. 3 is a fragmentary view similar to Fig. 1 showing a modified form of the safety control.

In the operation of hydraulic turbines, the flow of water from the penstock ID to the turbine is controlled by adjusting the positions of gates ll arranged in an annular series around the turbine and linked to a ring l2 which is connected through means including a crank l3 to the gate shaft [4. Booking of the latter to open and close the gates is usually effected by a hydraulic servo having a piston I5 reciprocable in a cylinder IS with its rod I'I connected to an arm [8 on the gate shaft l4. The flow of pressure fluid from a suitable source I9 to the head end of the cylinder I6 to open the gates or to the rod end to close the same is controlled herein by a relay valve 20 having a spool 2| whose lands, when the spool is centered, cover ports 22 and 23 leading through suitable passages to opposite ends of the servo cylinder. Pistons 24 and 25 on opposite ends of the stem of the valve spool 2| are slidable in cylinders respectively connected through suitable passages to ports 26 and 21 of a pilot valve 28. Normally, the latter is actuated automatically by the movements of flyballs 29 loaded by a selectively adjustable speeder spring 30 and pivoted on a ball head 3| which, in a well known manner, is driven in unison with the turbine shaft 32.

When the turbine speed rises above the speed setting of the governor, the flyballs lower the pilot valve stem 33 moving a land thereon below the port 21 thus permitting pressure fluid to flow to the cylinder of the piston 25. At the same time, a second land on the stem uncovers the port 26 thus connecting the cylinder of the piston 24 to the drain 34 thus permitting the valve spool 2| to be shifted to the left to a position determined by an adjustable stop 35. Pressure fluid is thus admitted to the rod end of the servo cylinder and released from the head end whereupon the piston I5 is moved to the right to initiate closing of the gates II. In a similar way, a fall in the turbine speed below the desired value raises the pilot valve stem 33 to admit pressure fluid to the piston 24 and through the relay valve 20 initiates gate opening movement of the servo actuator. The action of the governor as above described is modified by a well known compensating mechanism (not shown) preferably of the hydraulic type and actuated through a suitable mechanical linkage with the servo piston, the restoring force derived by this mechanism being applied to the pilot valve 28 but dissipated gradually by fluid leakage.

To avoid the danger of damaging the turbine, or the water conduit system between the dam and the turbine, by closure of the gates I I at too high a rate, provision is made, in the case of systems having long penstocks, for by-passing water out of the turbine casing 36 when the speed of the servo in closin the gates becomes excessive. This may be accomplished by a valve 31 controlling a by-passage 38 leading from the turbine casing directly to the turbine discharge 39. The movable member 49 of the valve 31 is normally urged to closed position against its seat by the pressure of water admitted from the turbine casing through a passage 4| and acting on a land 42 of the hollow valve member. The stem 43 of the latter projects from the valve casing and is pivotally connected intermediate the ends of a lever 44 having one end coupled through a linkage 45 to the rod of the servo piston so as to be moved down and up in unison with the opening and closing movements of the gates.

The other end of the lever 44 is pivoted at 45 on the outwardly projecting stem of a piston 46 slidable in a dashpot cylinder 41 pivotally supported at 46* and having opposite ends communicating with each other through a restriction formed by an adjustable needle valve 48.

If the gate closing movement of the servo occurs at a rate lower than that predetermined by the setting of the dashpot valve 48, the bypass valve 3! will remain closed and the servo motion transmitted through the linkage 45 will merely rock the floating lever 44 in a clockwise direction about the pivot 49. However, if the servo motion is at a higher rate, the dashpot will restrain the lever pivot 45 against downward movement so that at least part of the servo motion will result in swinging the lever 44 about this pivot thus raising the valve member 40 from its seat to cause by-passing of part of the delivered water around the turbine.

The by-pass valve 31 may come into action only occasionally under abnormal conditions occurring during shutting down of the turbine. Owing to such infrequent use, the valve may stick or be opened so slowly in relation to the servo motion as to be ineffectual in avoiding objectionable shock on the parts or damage to the turbine as the gates become closed. The present invention aims to detect such failure of the bypass valve to operate properly and, in such event,

to impose a supplemental control on the gate servo and reduce the rate of gate closure to a safe value.

Such partial disabling of the servo is effected in the present instance by a power actuator 50 acting on the relay valve 20 and activated by the action of a control device such as a valve Herein, the actuator is operated by fluid pressure and comprises a piston 52 slidable in a cylinder 53 which is alined with the outwardly projecting stem 54 of the relay valve. The piston 52 is urged outwardly by a spring 55 and the piston rod 56 projects from'the cylinder and carries the valve stop 35 above referred to, the latter being in the form of a screw held in the desired position of axial adjustment by a lock nut.

Normally, the parts are positioned as shown in Fig. 1, the stop 35 being retracted and inactive except for limiting the maximum leftward movement of the valve stem 54. When pressure fluid is admitted to the head end of the cylinder 53, the piston 52 is shifted to the right until it encounters a stop 51 as shown in Fig. 2. The stop 35 is thus projected forwardly against the stem 54 which is shifted to a position in which the land 58 on the spool 2| partially covers the port 22 thus limiting the rate at which pressure fluid may flow to the rod end of the servo cylinder I6. The extent of such restriction of the flow is determined by adjustment Qi H Stop 51.

Fluid is delivered to the auxiliary actuator through a passage 59 terminating in a port 60 in the cylinder of the valve 51 which port is normally partially covered by a land 62 on a stem 63 so that the passage 59 is connected to a drain outlet 95, thus allowing the spring 55 to retract the stop 35. A passage leading from the pressure source l9 communicates with the cylinder 6| at a point between the two lands on the valve stem.

The failure of the by-pass valve 31 to function properly during rapid closure of the turbine gates II is detected in the present instance by combining the separate motions of the gates or servo and the by-pass valve member 40 in a novel manner and applying the resultant force to a device which is indicated generally at 64 and which acts selectively to difierentiate between the normal operations of the gate servo and bypass valve but operates the pilot valve 5| in the intended manner under those abnormal conditions which require partial disabling of the gate servo in order to avoid the danger of the turbine and water conduit system being damaged as the gates become fully closed. Preferably, the differentiating device 54 is of hydraulic character comprising a chamber 65 completely filled with fluid and communicating through an adjustable needle valve 66 with an adjacent open chamber 6! partially filled with fluid. A plunger 68 on a stem 69 is slidable through the dividing wall 10 in and out of the chamber 65 to vary the pressure of the fluid therein above or below atmospheric pressure and thus exert outwardly or inwardly directed forces on a second or receiving piston H also slidable through the wall 10. The piston H is on a rod 12 connected through a lever 13 to the stem of the valve 63 and is urged toward the normal position shown by a tension-compression spring 14 coiled about the piston rod. The range of motion of the receiving piston H in opposite directions away from the normal position is limited by cutting away opposite end portions as indicated at 15 and 16 thus allowing fluid to be by-passed between the chambers 65 and 6! whenever the piston moves beyond the limits of a range within which the valve 5| is actuated in the manner above described.

In the form shown in Fig. 1, the motions of the gate servo and the by-pass valve 31 are combined by means of a floating lever 11 pivotally connected intermediate its ends at 18 to one end of a link 19 whose other end is connected through lever 86 to the rod 69 of the piston 68. One end of the floating lever 11 carries the follower 8| of a cam 82 having a gradual rise and fast on a rockshaft 83 carrying an arm 84 which is connected by a link 85 to the piston rod ll. As the servo piston moves back and forth to open and close the gates II, the cam 82 swings in unison clockwise and counterclockwise respectively, the left end of the lever 11 being raised during the closing movement.

A similarly shaped cam 86 is fast on a rockshaft 81 and acts on a follower '88 at the other end of the lever 11. Through an arm 89, a link 90, and a lever 9|, this cam is connected to the stem 43 of the by-pass valve 31 so that the cam is oscillated clockwise and counterclockwise respectively during the opening and closing movement of the valve member 40.

Under normal conditions, that is when the gates are moving back and forth under the control of the gQVcrnor, the corrective motions may be so slow that the by-pass valve 31 remains closed and the cam 86 is stationary. This is for the reason that fluid leaks through the dashpot restriction 48 and permits free movement of the piston 46 at a rate greater than the rate of the servo motion. The gate closing motion, is, however, accompanied by counterclockwise swinging of the cam 82 thus raising the lever I1 about the pivot 92 as a fulcrum, and this results in downward movement of the actuating piston 68 of the hydraulic device 64. This tends to compress the fluid in the chamber 65, but the rate of displacement is substantially less than the rate at which fluid may flow through the needle valve 66, the setting of which is adjusted to permit such escape of the fluid. As a result, the pressure on the receiving piston H is not increased sufliciently to overcome the spring 14 and move the valve stem 63 downwardly. The auxiliary actuator 5|] thus remains inactive and the rate of servo motion is not reduced.

The supplemental control also remains inactive if, during more rapid closing of the gate, the bypass valve member opens properly and the opening continues at the required rate predetermined by the setting of the dashpot needle 48. In such a case, the cam 86 is turned clockwise in unison with the opening of the by-pass valve thereby allowing the pivot 92 of the lever 11 to fall and thus neutralize the rise of the follower 8| by the servo actuated cam 82. In this case substantially no downward movement of the transmitting piston 68 occurs, and, therefore, the pilot valve 5| remains open to drain.

Assume now that the by-pass valve member 40, due to prolonged idleness or otherwise, sticks and fails to open properly at a time when the gates are being closed at a high rate which might result in damage to the turbine, or water conduit system. In such a case, the cam 86 is not turned and the downward movement of the transmitting piston 68 caused by turning of the cam 82 will tend to displace fluid from the chamber 65 at a rate greater than the permissible flow through the restriction 66. The pressure thus exerted on the receiving piston H overcomes the spring 14 and moves the stem 63 to lower the land 62 of the valve 5! below the port 60 and thus admit pressure fluid to the actuator 50. Thereupon, the iston 52 is shifted against the stop 51 and the stop 35 engages the stem 54 of the relay valve and shifts the latter to the position shown in Fig. 2 in which the port 22 is now only partially covered, thus restricting the flow of pressure fluid to the rod end of the servo cylinder. The speed of the servo is thus reduced to a safe value which is maintained throughout the remaining closing movement of the gates if the by-pass valve continues to remain closed. After the gates become fully closed or the motion of the cam 82 is otherwise interrupted, fluid is no longer displaced from the chamber 65 and the receiving piston H is permitted to return downward to its normal position under the action of the spring 14. The resulting osition of the land 62 connects the cylinder '53 to the drain line thus deenergizing the actuator so as to restore the relay valve 20 to the exclusive control of the governor.

It will be apparent that the limiting action above described may also take place if the by-pass valve 31 opens and provided that the rate of such opening is too slow to allow closure of the gates at the rate determined by sto 35. In this case, the cam 82 turns faster than the cam 86 and the diiference in the two rates causes a constant motion of the piston 68 and displacement of fluid from the chamber at a rate greater than the permissible leakage through the valve 66. Thus, the limiting mechanism operates not only to detect a complete failure of the by-pass valve to open properly, but also the failure of this valve to open at a sufficiently rapid rate to avoid possible damage to the turbine and water conduit system.

The supplemental rate limiting mechanism is, it will be observed, ineffectual during opening of the gates. Then the cam 82 turns clockwise and raises the transmitting piston 68 thereby creating a partial vacuum in the chamber 65. This draws the receiving piston II inwardly but only until the shoulder 15 is uncovered allowing fluid to by-pass freely into the chamber 65. The accompanying upward movement of the valve stem 63 is not sufficient to carry the land 62 past the fluid supply passage so that the actuator 50 remains deenergized.

Instead of utilizing a mechanical means such as the cams 82 and 8B and the lever 11 above described to combine the motions of the gates and the by-pass valve, the combination may be eifected hydraulically and within the differentiating device 64 itself. Such a modification is shown in Fig. 3 wherein the receiving piston 1| is formed directly on the stem 63 of the pilot valve 5|. Opposite ends of the tension compression spring 14 are attached respectively to the cover of the valve 5| and the stem 63 in such a manner that the lower land 62 of the valve partially covers port 60 so that passage 59 is normally connected to drain 95. Two transmitting pistons 68 and 68 are extended through the wall 70 into the chamber 65 and urged outwardly by individual springs. An arm 93 on the rockshaft 83 bears against the outer end of the piston 68 so that the gate closing movement of the servo is transmitted to this piston to permit the latter to move outwardly under the action of its spring 68. In a similar way, the opening movement of the by-pass valve member 40 is transmitted through the link and a motion-reversing lever 90 to the arm 89, the motionbeing applied to the piston 68 by the arm 94 on the rockshaft 81 to move the piston inwardly.

The operation of the modified construction is similar to that form first described above. In response to closing of the gates at an objection ably high rate, the fluid pressure in the chamber 65 will drop sufiiciently to cause the stem 63 to be lowered by atmospheric pressure whenever fluid is displaced from the chamber at a sufliciently high rate, either by outward movement of the piston 68 alone, or by outward movement at a higher rate subtracted from the displacement due' to the inward movement of the piston 68' during opening of the by-pass valve. Thus, the limiting mechanism remains inactive under normal operating conditions but detects and responds to failure of the by-pass valve to function properly under conditions which might otherwise result in damage to the tubrine and water conduit system.

We claim as our invention:

1. The combination of a hydraulic turbine having movable gates, an automatic by-pass valve, and means for opening said by-pass valve as the gate closing movement exceeds a predetermined rate, a hydraulic servo having a piston for actuating said gates, a governor driven in unison with said turbine, a relay valve controlled by said governor and having a member movable to opposite positions, a fluid system for receiving pressure fluid delivered through said valve and directing the same to opposite ends of said piston in accordance with the position of said valve member, a power actuator associated with said member and operable when rendered active during closing of said gates to shift the member to reduce the rate of fluid flow to said servo and limit the rate of further gate-closing movement of said servo to a safe value, a normally inactive control valve controlling the energization of said actuator and having a movable element operable when moved out of inactive position to a predetermined position to cause activation of said actuator, a fluid filled chamber having a restriction through which fluid may escape and return to the chamber at an adjustable rate, a piston connected to said valve element and operable to move the same to said predetermined position in response to a pressure change in said chamber to a predetermined value, means to restore said piston to its normal position when said pressure change is dissipated, piston means projecting into said chamber, a connection operable to transmit the gate-closing motion of said servo piston to said piston means and move the latter into said chamber, and a separate connection operable to transmit the opening movement of said by-pass valve to said piston means and correspondingly retract the piston means out of the chamber whereby to vary the pressure in said chamber in accordance with the difference between the rates of such movements.

2. The combination of a hydraulic turbine having movable gates, an automatic by-pass valve, and means for opening said by-pass valve as the gate closing movement exceeds a predetermined rate, a hydraulic servo having a piston for actuating said gates, means driven in unison with said turbine and including a governor controlled relay valve having a movable member, said means operating selectively to admit pressure fluid to opposite ends of said piston in accordance with changes inthe position of said member, a power actuator operable when rendered active to shift said member to a position in which the rate of fluid flow to said servo is restricted and the rate of gate closing movement of said servo limited to a safe value, a device controlling the energization of said actuator, a fluid filled chamber having a restriction through which fluid may escape and return to the chamber at an adjustable rate, a piston connected to said device and movable from a normal position to actuate the same in response to a pressure change in said chamber to a predetermined value, means normally urging said piston and device to said normal position, piston means projecting into said chamber, and connections between said servo piston and said by-pass valve for transmitting the gate closing motion of the servo piston to said piston means to move the latter into said chamber and to transmit the opening movement of said by-pass valve to said piston means to retract the piston means out of the chamber whereby to vary the pressure in said chamber in accordance with the difference between the rates of such movements.

3. The combination of a hydraulic turbine having movable gates, an automatic by-pass valve, and means for opening the by-pass valve as the gate-closing movement exceeds a predetermined rate, a reversible power servo for actuating said gate, means driven in unison with said turbine and including a governor controlled member movable selectively to regulate the energization of said servo to determine the direction and extent of movement of the gate in response to changes in the speed of the turbine, a power actuator operable when rendered active to shift said member to a position for reducing the rate of the gate closing movement, a device controlling the energization of said actuator and operable to effect such energization when moved to a predetermined position, a fluid filled chamber having a restriction through which fluid may escape and return to the chamber at an adjustable rate, a piston connected to said device and operable in response to a pressure change in said chamber to a predetermined value to move out of a normal position and thereby move the device to said predetermined position to activate said actuator, piston means projecting into said chamber, and connections extending from said servo piston and said by-pass valve and operable to transmit the gate-closing motion of said servo to said piston means to move the latter into said chamber and to transmit the opening movement of said bypass valve to said piston means to retract the piston means out of the chamber whereby to vary the pressure in the chamber in accordance with the difierence between the rates of such movements.

4. The combination of a hydraulic turbine having an automatic by-pass valve, and means for opening the by-pass valve when the gate closing movement exceeds a predetermined rate, a reversible power servo for actuating said gates, a power actuator operable when rendered active to limit the degree of energization of said servo in a direction to close said gates, a device having a normal position and movable therefrom to control the energization of said actuator, a fluid filled chamber having a restriction through which fluid may escape and return to the chamber at an adjustable rate, a piston connected to said device and operable in response to a pressure change in said chamber to a predetermined value to move out of an inactive position and thereby move the device out of a normal position to a predetermined position and thereby activate said actuator, piston means projecting into said chamber, and means for receiving the gate-closing motion of said servo and transmitting the same to said piston means to move the latter into said chamber and also for receiving the opening movement of said by-pass valve and transmitting the same to said piston means to retract the piston means out of the chamber and thereby vary the pressure in the chamber in accordance with the difference in the rates of such movements.

5. The combination of a hydraulic turbine having movable gates, an automatic by-pass valve, and means for opening the by-pass valve as the gate closing movement exceeds a predetermined rate, a reversible power servo for actuating said gates, means driven in unison with said turbine and having a governor controlled member movable selectively, said means operating to regulate the energization of said servo to determine the direction and extent of movement of the gates in response to changes in the speed of the turbine, a power actuator having a normal position and operable when rendered active to move out of said position and shift said member to a position for reducing the rate of the gate-closing movement, a fluid filled chamber having a restriction through which fluid may escape and return to the chamber at an adjustable rate, means ineluding a piston urged to a normal inactive position and operable in response to a change in the pressure within said chamber to a predetermined value to move out of said inactive position and activate said actuator, piston means projecting into said chamber and movable to displace fluid therein, a connection between said piston means and said servo and operable to move the piston means in onedirection in unison with the gate-closing movement of the servo, and a connection between said by-pass valve and said piston means operable to move the latter in the opposite direction during opening of the by-pass valve whereby to vary the pressure in said chamher in accordance with the difference between the rates of the gate closing and valve opening movements.

6. The combination of a hydraulic turbine having movable gates, an automatic by-pass valve, and means for opening the by-pass valve as the gate closing movement exceeds a predetermined rate, a reversible power servo for actuating said gates, means driven in unison with said turbine and having a governor controlled member movable selectively, said means operating to regulate the energization of said servo to determine the direction and extent of movement of the gates in response to changes in the speed of the turbine, a power actuator having a normally inactive position and operable when rendered active to move out of said inactive position and shift said member to a position for reducing the rate of the gate-closing movement, a fluid filled chamher having a restriction through which fluid may escape and return to the chamber at an adjustable rate, means including a piston normally urged to a predetermined position and operable in response to a change in the pressure within said chamber to a predetermined value to activate said actuator, and mechanism actuated by and in unison with the gate-closing movement of said servo and the opening of said bypass valve and operable to vary the pressure in said chamber in accordance with the difference in the rates of the gate closing and valve opening movements.

7. The combination of a hydraulic turbine having movable gates, an automatic by-pass valve, and means for opening the by-pass valve when the closing movement of the gates exceeds a predetermined rate, a reversible power servo for actuating said gates, means having a member movable selectively and operable to regulate the energization of said servo to determine the direction and extent of movement of the gates in response to changes in the speed of the turbine, a normally inactive power actuator operable when rendered active to modify the action of said regulating means by shifting said member to a position for reducing the rate of the gateclosing movement, a hydraulically acting differentiating device having a movable transmitting element, mechanisms connecting said device and said gates and said by-pass valve and operable in response to the gate-closing movement of said servo and the opening movement of said by-pass valve to change the resultant pressure in said device correspondingly, said device having a receiving element movable in response to a pressure change in said device to a predetermined value, and means actuated by said receiving element and controlling the activation of said actuator.

8. The combination of a hydraulic turbine having movable gates and an automatic by-pass valve, and means for opening the by-pass valve when the closing movement of said gates exceeds a predetermined rate, a reversible power servo for actuating said gate, means for regulating the energization of said servo to determine the direction and extent of movement of said gates in response to changes in the speed of the turbine, a normally inactive power actuator operable when rendered active to modify the action of said regulating means and reduce the rate of the gate-closing movement by said servo, a differentiating device actuated by the gate-closing movement of said servo and the opening movement of said by-pass valve and operable to produce a resultant motion at a rate corresponding to the difierence in the rates of the gate closing and valve opening motions, a hydraulic device having a fluid filled chamber, means for displacing fluid in said chamber in accordance with the magnitude of said resultant motion, and mechanism operable to detect a pressure change in said device to a predetermined value and thereupon activate said actuator.

9. The combination of a hydraulic turbine having movable gates, an automatic by-pass valve, and means for opening the by-pass valve when the rate of closure of said gates exceeds a predetermined value, a reversible power servo for actuating said gates, means for regulating the energization of said servo to determine the direction and extent of movement of said gates in response to changes in the speed of the turbine, a motion difierentiating device responsive to the gate-closing movement of said servo and the opening movement of said by-pass valve and operable to combine such motions and produce a resultant motion corresponding to the difierence in the rates of said gate-closing and valve opening motions, and means actuated by said device in accordance with the change in said resultant motion and operable in response to an increase in the rate of the resultant motion above a predetermined value to modify the action of said regulating means and limit the rate of gateclosing movement of said servo.

10. The combination of a hydraulic turbine having movable gates and an automatic by-pass valve, a reversible power servo for actuating said gates, means for regulating the energization of said servo to determine the direction and extent of movement of the gates, a fluid filled chamber having a restriction through which fluid may escape and return to the chamber at an adjustable rate, a differentiating device including transmitting and receiving piston means movable into and out of said chamber to vary the pressure therein, means responsive to the movement of said receiving element to a predetermined position to reduce the speed of the gateclosing movement of said servo to a predetermined value, and separate connections respectively Joining said gate servo and said by-pass valve to said differentiating device and operable to actuate said transmitting piston means in accordance with the difference between the rates of gate-closing movement of said servo and the opening of said by-pass valve.

11. The combination of a hydraulic turbine having movable gates and an automatic by-pass valve, a reversible power servo for actuating said gates, a speed governor driven in unison with said turbine and selectively regulating the energization of said servo to determine the direction and extent of movement of said gates in response to changes in the speed of the turbine, a normally inactive power actuator operable when rendered active to modify the action of said governor and reduce the rate of the gate-closing movement by said servo, a hydraulically acting difierentiating device having a fluid filled chamber and two movable transmitting pistons movable into and out of the chamber, mechanisms respectively movable in accordance with the gateclosing movement of said servo and the opening movement of said by-pass valve and operable to transmit such movements to the respective transmitting pistons to change the resultant pressure in said device at corresponding rates, said device having a receiving element movable in response 12 to a pressure change in said device to a predetermined value, and means actuated by said receiving element and controlling the activation of said actuator.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 10 1,309,808 Pfau July 15, 1919 1,706,813 Pfau Mar. 26, 1929 2,059,649 Pfau Nov. 3, 1936 2,343,223 Pfau Feb. 29, 1944 

